I had to buy more aluminium than needed but the total cost with right quantities is about 50-70 Euro.
as a test, 80 kg, about 175 pounds were smoothly transported.
Step 1: What is needed
4 x 1mt aluminium corner profile - 2 of them with sides cuts at 45°
4 x 80cm aluminium corner profile - 2 of them with sides cuts at 45°
1 x aluminium square pipe (40mm wide)
10 x tropicalized brackets (4 flat 90°, 4 vertical 90°, 2 linear
4 x corner brackets
about 2m of iron square pipe 30mm wide (that replaced the round pipe in the image that was not working good)
beads (I used the 2cm thick ones)
rivets, screws, bolts and nuts
hand made gimbal
Step 2: Frame
Rivet the 80mm aluminium profile (straight cut) at the center of the external long side to get a Z shaped side.
Add a 90° tropicalized bracket at the center of the bottom profile and grind it to fit in the profile. Cut with a grinder the center hole of the brackets to get a fork where you will fix the wheel hub.
Rivet the 1mt aluminium profile with the straight cut to the frame, at the bottom side, internally, leaving enough space to fit the wheel.
On these profiles, you have to rivet the other two 90° brackets making the same work of the previous.
Add longitudinally the square aluminium pipe at the center of the frame
Step 3: Wooden parts
Always with the beads, I shaped the wheel fenders, fixing them to the base and the sides with wood screws.
Step 4: Trailer bar
Insert the iron pipe into the aluminium pipe for about 50cm, then drill and add passing bolts. This will fix the trailer bar to the frame. If there's a little play between the two pipes, it's not a big problem, but if you want you can add some gauge between them and then add bolts, both on the vertical axis and on the horizontal axis.
Fix the corners with the remaining brackets (90° flat and linear) to give strenght to the whole bar